Faraday-effect device for electro-magnetic waves



y 9,1959 c. LQHOGAN 2,887,664

FARADAY-EFFECT DEVICE FOR ELECTROMAGNETIC WAVES Original Filed Oct. 22,1951 3 Sheets-Sheet 1 FIG.

R0721 TABLE JOIN T SOURCE OF MODULA T/NG POTENTIALS MON TOR/N6 DETECTORINVENTORI C. L. HOGAN ATTORNEY May 19, 1959 c. L. HOGAN 2, 7,

' FARADAY-EFFECTDEVICE FOR ELECTROMAGNETIC WAVES Original Filed Oct. 22,1951 3 Sheets-Sheet 2 ANTENNA C/RCUl-T "4" (MAY BE OMITTED) 4oTRANSM/TTER RECEIVER as (l 'RT/CALLV "c POLAR/ZED) I a/l FIG. 6

FIG. 7

r0 PULSER I d 6 lNVENTOR C. L. HOGAN A TTORNEV May 19, 1959 HOGAN2,887,664

FARADAY-EFFECT DEVICE FOR ELECTROMAGNETIC WAVES Original Filed on. 22,1951 FIG. 9

v3 Sheets-Shee t 3 a To CIRCUIT "c CIRCUIT 'FIG. RO7I472IBLE d FIG.JOINT FIG l2 32o osnsre'os ROTATION /o40 OERSTEDS ROTATION FARADAVROTATION PER CM. PATH LENGTH DEGREES l l l l 1 O 200 400 600 800 I000I200 140016001800 2000 STRENGTH 0F APPLIEQMAGNET/C FIELD (OERSTEDS) INl/E N TOR c. L. HOGAN ATTORNEY United States Patent FARADAY-EFFE'CTDEVICE FOR ELECTRO- MAGNETIC WAVES Clarence L. Hogan, Lexington, Mass.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. acorporation of New York Original application October 22, 1951, SerialNo. 252,432,

now Patent No. 2,748,353, dated May 29, 1956. Di-

vided and thisapplication September 13, 1955, Serial No. 534,089 g 12Claims. (Cl. ass-9 This application is a division of my priorapplication Serial No. 252,432, filed October '22, 1951, now UnitedStates Patent No. 2,748,353, granted May 29, 1956, which is in turn adivision of my application Serial No. 228,379, filed May' 26, 1951,..nowUnited States Patent No. 2,768,354, granted October 23, 1956. V

This invention relates to new and useful devices employingFaraday-effect rotation of the plane of polarization of plane polarizedelectromagnetic waves.

An object of the invention is to extend practical usefulness of theFaraday effect into the range of wavelengths longer than those of thevisible spectrum and particularly into the microwave range, withoutproducing material attenuation of the waves in the process.

In accordance with the invention, in certain specific embodimentsthereof, a block of magnetic material, for example nickel-zinc ferritewhich maybe in the form of a cylinder a centimeter in diameter and acentimeter thick, more or less, is used in the microwave range as ananti-reciprocal (Faraday-effect) rotator of the plane of polarization ofplane polarized electromagnetic waves and as such may be employed inswitching devices, attenuators, modulators, circular polarizers, etcetera, and in the class of devices not governed by a reciprocity law,for example, one-way transmission systems, and directionally selectivephase shifters including certain devices which have been calledgyrators.

The Faraday effect in the optical range has long been known. Attemptsheretofore to extend the use of the effect to wavelengths longer thanthose of the visible spectrum, and in particular to the microwave rangehave been of no practical consequence because of the general weakness ofthe Faraday effect in substances that are transparent to visible lightor because of excessive attenuation of waves of longer wavelength suchas microwaves in suitable thicknesses of certain ferromagneticsubstances such as iron where the amount of Faraday-effect rotation perunit thickness for very thin sheets that could be penetrated by lightwas known to be large. Published researches on the theory offerromagnetic resonance have provided a theoretical basis from which Ihave been able to predict the presence of a pronounced Faraday effect insubstances which exhibit this kind of resonance, for example,ferromagnetic metallic oxides. I have discovered that magnetic materialssuch as the ferromagnetic metallic oxides and notably nickel-zincferrite, in a thickness of the order of magnitude of a wavelength, arecapable of producing angular rotations of the plane of polarization ofplane polarized waves to the extent of 45 degrees or 90 degrees or morein the presence of magnetizing fields which are readily produced inpractice and that these materials in such thicknesses are capable oftransmitting electromagnetic waves, for example in the centimeter range,with substantially negligible attenuation. I have discovered furtherthat the angle of rotation of polarized electromagnetic waves inmagnetic materials exhibiting ferromagnetic resonance is approximatelydirectly proportional to the thickness of the material traversed by thewaves and to the intensity of magnetization to which the material issubjected, whereby it is possible to adjust the amount of rotation byvarying or properly choosing the thickness of the material traversed andthe intensity of magnetization.

This application is directed to arrangements utilizing an angularrotation of 90 degrees in the direction of the plane of polarization.

In the drawings:

Fig. 1 is a perspective view of a piece of magnetic material usefulsingly or in combination as an embodiment of the invention;

Fig. 2 is a view, partly in perspective and partly diagrammatical,showing a system utilizing a piece of material such as shown in Fig. 1in a wave guide system for microwaves;

Figs. 3, 4, and 5 are views, partly in perspective and partlydiagrammatical showing other embodiments of the invention, Fig. 3showing an antifading system, Fig. 4 a system for producing circularlypolarized waves, and Fig. 5, a nonreciprocal four channel switchingarrangement;

Figs. 6 and 7 are diagrams useful in explaining the operation of theequipment of Fig. 5;

Fig. 8 is a view partly in perspective and partly diagrammatical showingan embodiment of the invention i a one-way transmission system; I

Fig. 9 is a view, partly in perspective and partly diagrammatical,showing a combination of wave guide junctions with a 90-degree phaserotator to form a nonreciprocal four channel switching arrangement whoseproperties are similar to the system shown in Fig. 5;

Fig. 10 is a diagram useful in explaining the operation of the system ofFig. 9;

Fig. 11 is a perspective view of a wave guide system embodying theinvention, mounted in a cylindrical opening in a permanent magnet orother magnetic core structure; and

Fig. 12 is a plot of the relationship between strength of appliedmagnetic field and Faraday-effect rotation in a certain elementembodying the invention.

Referring to Fig. 1, there is shown a block 1 of magnetic material, forexample a piece of ferromagnetic me tallic oxide, capable oftransmitting electromagnetic waves and having a thickness of the orderof magnitude of a wavelength, for example one centimeter, more or less,which has been found to operate satisfactorily as a directionallyselective phase rotator for polarized electromagnetic waves. An elementof this type which has been successfully used was made by powderingsintered nickelzinc ferrite until the largest particles passed a 40-meshscreen, mixing this powder with small spheres of polystyrene which werefine enough also to pass a 40-mesh screen, in the proportion of 75percent of volume of ferrite and 25 percent volume of polystyrene, thenslightly wetting the mixture with a dilute solution of polystyrene inbenzene to render the mixture tacky and pressing the material into acylindrical form in a die under five tons pressure. By measuring thedensity before and after powdering it was estimated that the'finalcompacted cylinder was approximately 60 percent nickel-zinc ferrite byvolume. Before powdering, the nickel-zinc ferrite had been determined tohave a saturation magnetization of approximately 2250 gauss and aresistivity of about ten million ohm-centimeters and hence the cylinderas formed dered cylinder should be only six-tenths of that'in'theoriginal sintered nickel-zinc ferrite. Since mathematical analysis showsthat the rotation per centimeter is approximately proportional to thesaturation magnetization, no apparent advantage in degree of rotation isobtained by powdering. However, the tapered transition members (2 and 3in Fig. 2) that were used to cut down reflections from the faces of theferrite cylinder were made of polystyrene and it was advantageous tohave the dielectric constant of the composite ferrite cylinder matchthat of the transition members as closely as possible. This match isconsiderably improved by the powdering and dilution technique discussedabove.

In order to etfect Faraday rotation, waves must be transmitted through atransparent isotropic medium parallel to the direction of the lines offorce of the magnetic field. The eifect may be conveniently produced byplacing the medium along the axis of a solenoid. The rotation isdesignated as positive if it is in the direction of the posi tiveelectric current which produces the magnetic field and negative if inthe opposite direction. It is believed that all transparent substancesshow the Faraday rotation. It was at one time though that diamagneticsubstances showed a positive rotation and ferromagnetic substancesshowed a negative rotation but this is now known to be incorrect. Whilenot explicitly stated, a theory of the ferromagnetic Faraday effect canbe derived from an article by D. Polder in Philosophical Magazine,volume 40, pages 99-115 (1949), in connection with an analysis of thegyromagnetic resonance in ferromagnetics involving precession ofspinning electrons about the direction of an impressed magnetic field.

In the above-mentioned reference, Polder has shown that anelectromagnetic wave being propagated in a ferromagnetic medium which ishomogeneously magnetized in a direction parallel to the direction ofpropagation will split into positive and negative circularly polarizedcomponents which travel with different velocities in the medium.

In deriving his expressions, however, Polder assumed no damping of theelectrons as they precess about the field direction. This assumption isprobably valid at frequencies which are far removed from the resonanceabsorption frequency. In addition, however, Polder has tacitly assumedno dielectric loss as the wave is propagated through the ferromagneticmedium. Since appreciable dielectric loss occurs when microwaves arepropagated through ferrites it is necessary to extend Polders theory toinclude this case. In addition, Polders theory is extended below toexplain the ferromagnetic Faraday effect in the vinicity of thegyromagnetic resonance.

If an effective magnetic field, H is applied in the-Z direction, toa'ferromagnetic medium, and if then a high frequency field is applied inan arbitrary direction, the relation between the periodic parts of B andH (namely band h), is given by thefollowing equations:

m =gyromagnetic ratio for electrons=17.6 radians per secondv per oerstedw=angular frequency of the incident wave in radians per second M=magnetization of medium.

If a planeelectromagnetic wave is propagated through "thisme'dium in theZdirection it is necessar in'order'to describe this wave, to find a setof solutions to Maxwells equations which are consistent with the aboveset of Equations 1, 2, and 3, and in which b, h, E, and D are allproportional to exp(jwt-IZ) It can be shown that the only. possiblesolution under these conditions consists of either apositive or anegative circularly polarized wave for'which the propagation constantsare:

If Equations 4 and 5 are solved for the phase constants 5 and {L thefollowing result is obtained:

where e=e'-] e" =a. complex dielectric constant,

and

From this relation, one can obtain a real index of refraction for eachcomponent, given by Since the electromagnetic wave under discussionsplits into two circularly polarized components which travel withdifferent velocities in the ferromagnetic medium, the components will,upon emerging from the medium, unite to form a plane polarized wave inwhich the'plane of polarization has been rotated through an angle 0while traveling through the medium, where in which l=path length throughthe ferromagneticmedium By means of Equation 6 above, this can beWritten as If the'effective -magnetic field within theferromagneticmedium is small enough so that Equation 9 canbe written as If we assumeas reasonable representative values:

41rM =1000 gauss,

Equation13 gives a rotation of & 69 degrees per centimeter (18) Animportant result of the foregoing analysis is that this relatively largerotation appears at frequencies greatly different from the resonanceabsorption frequency; and, if the conditions and (12) obtain, therotation as expressed in Equation (13) is seen to be independent of thefrequency of the incident wave, and hence any device making use of thisrotation would be broadband in its transmission characteristics. Inaddition, the rotation is proportional to the magnetization of thesample. Below magnetic saturation, the magnetization is, of course,dependent upon the applied magnetic field, and hence, below saturation,the rotation is substantially proportional to the applied magneticfield.

If magnetic losses are included in the analysis, the theory alsopredicts that in the region where the frequency of the incident wave isclose to the ferromagnetic resonance absorption frequency, the positivecircularly polarized component is substantially completely absorbedwhile the negative component is propagated with attenuation onlyslightly difierent from that due to the usual dielectric losses. Hence,near the resonance absorption frequency the wave after transmissionthrough the ferromagnetic medium is circularly polarized.

It has been experimentally confirmed that this theory does predictquantitatively the behavior of most ferrites up to saturation. Inaddition, I have obtained substantially circularly polarized waves bytransmitting a plane polarized wave through a block of ferrite when thefrequency of the Wave was close to the resonance frequency. Theaccompanying transmission loss measured only slightly greater than threedecibels, indicating that the negative component was being propagatedwith only slight attenuation while the positive component was beingalmost completely absorbed.

It should be noted that the Faraday rotation depends for its directionupon the direction of the magnetic field, in the same manner as thedirection of translation of a screw is related to its direction ofrotation. Thus, if the direction of the magnetic field is reversed, thedirection of the Faraday rotation is also reversed in space whileretaining its original relationship to the direction of the field. Therotation is independent of the sense of propagation along the axis ofthe ferromagnetic element. A wave passing through the element first inone direction and then in the other undergoes two successive phaserotations in the same sense, thereby doubling the rotation undergone ina single passage. This differs from the case of a phase rotationobtainable in a twisted rectangular wave guide, for example, where awave passing through in one direction has its plane of polarizationrotated through a certain angle but upon passing through in the reversedirection has its plane of polarization rotated back through the sameangle to its original orientation. vThe same distinction holds forrotations undergone by a wave passing through certain crystallinestructures, wherein a plane polarized wave after passing back and forththrough the same structure an even number of times undergoes no netrotation of its plane of polarization.

In a simplified view of the phenomenon involved, a plane polarized waveincident upon the magnetic material in the presence of the magneticfield produces two sets of secondary Waves in the material, each set ofsecondary waves being circularly polarized. The two sets of secondarywaves are circularly polarized in opposite senses and they travelthrough the medium at unequal which is in general polarized at adifferent angle from the original wave.

Fig. 2 shows the element of Fig. 1 arranged to be used as either avariable attenuator or as a modulator.

speeds. Upon emergence from the material, the secondary waves incombination set up a plane polarized wave,

The element 1 with accompanying conical transition members 2 and 3 whichmay be of polystyrene, is shown mounted inside a length of circular waveguide 4 which is in turnrnounted between two aligned and similarlyoriented circular to rectangular transition wave guide members 5 and 6.By similar orientation is meant that the long dimension of therectangular guide in section 5 is parallel to that in section 6 andsimilarly the short dimension of the section 5 is parallel to the shortdimension of the section 6. The Wave guide members 4, 5, and 6 may bejoined together by means of flanges, one or more of which may berotatable, or in other suitable manner known to the prior art. Asolenoid 7 is mounted upon the outside of the wave guide section 4 andis supplied with a source 8 of energizing current represented forpurposes of illustration as a battery shunted by a potentiometer 9having a variable contact arm 10. A source of modulating potentials 11is arranged to be connected in series with the solenoid 7, when desired,by means of a double-pole double-throw switch 12. With the switch 12 inthe lower of the positions shown in Fig. 2 the source 11 is connected tothe solenoid 7 through a suitable transformer 13. When the switch 12 isin the upper position the source 11 is disconnected and the circuit ofthe solenoid is completed through a strap 14. In an embodiment that hasbeen successfully operated, the solenoid 7 had a length of about onefoot and the element 1 was placed in the uniform field region in thecenter of the coil.

In the operation of the system of Fig. 2 as a variable attenuator, theswitch 12 is thrown to the upper position.

The rectangular portions of the wave guide sections 5 and 6 serve aspolarizers for plane waves in that they will accept only that componentof the electric vector which is consistent with the TB mode in therectangular guide, and by means of a smooth transition from rectangularto circular cross section, this mode goes over into the TE mode in thecircular portion of the guide. The dimensions of the wave guides arepreferably chosen so that only the dominant mode in each can bepropagated. The solenoid 7 when energized provides an axial magneticfield in the direction of the thickness of the device 1. The wave guidesections 5 and 6 being aligned and similarly oriented will accept onlythe component of the electric vector in one preferred direction. Withthe arm 10 set at the extreme right-hand end of the potentiometer 9there is substantially no magnetizing potential applied to the solenoid7 and, assuming that there is no residual magnetization, no rotation ofthe plane of polarization occurs in element 1. A wave of suitablefrequency received from an oscillator and impressed upon the section 5passes through the element 1 and into the section 6 without any changein the plane of polarization and with only a small amountof attenuationwhich is inherent in the operation of the element 1 which in anembodiment that has been successfully oper'ated Was found to be of theorder of magnitude of 0.8 decibel at a wavelength of approximately threecentimeters. Alignment of the sections 5 and 6 may be adjusted by meansof the rotatable joint 60. Attenuation may be introduced into thesystem. between the sections 5 and 6 by moving the potentiometer arm 10to the left thereby supplying an adjustable amount of magnetizingcurrentto the solenoid 7, and the potentiometer 9 may be calibrated inknown manner to indicate in decibels or other suitable units the amountof attenuation inserted at any setting of the arm 10. The addedattenuation comes about on account of the rotation of the plane ofpolarization of the Waves which is accomplished due to the Faradayeffect in the element 1 thereby turning the plane of polarization of thewaves away from the preferred direction in the section 6 so that. only acomponent of the fullintensity of the Wave is effective to produce anoutput in the seetion..6. The component that is rejected is absorbed bythe resistive vanes 46 and 47 which are preferably notched in order toafford a smooth transition and insure maximum absorption. When theelement 1 introduces a 45-degree rotation, the intensityof the output ofthe section 6 has has approximately one-half the value which it wouldhave in the preferred polarization and approximately three decibels ofattenuation are added thereby. When a 90-degree phase rotation isintroduced the wave is polarized at right angles to the preferreddirection in. the section 6 and substantially no output is produced. Themaximum attenuation that might be introduced is theoretically infiniteand in the practical case may be made very large by the use of amoderately strong magnetic field such as approximately 250 gauss. Thetransition members 2 and 3 serve to reduce reflections of waves thatwould arise due to the discontinuity introduced by the surfaces of theelement 1. The output of the section 6 may be fed to an antenna or anyother suitable utilization device.

Turning now to the operation of the system of Fig. 2 as a modulator, theswitch 12 is to be thrown to the lower position and the potentiometerarm 10 may be set permanently at an intermediate position on thepotentiometer. In such a position and in the absence of modulatingpotentials an intermediate amount of phase rotation is introduced by thesolenoid 7 to the device 1, a suitable amount being, for example, 45degrees. The normal output of the section 6 is in this case reducedaccordingly to approximately-one-half. Application of modulatingpotentials from the source 11 serves to vary the magnetizing current inthe solenoid and thereby altern'ately to increase and decrease theamount of phase rotation produced and consequently the output of thesection 6, in accordance with the variation of the modulatingpotentials. In this way a modulated wave is produced and may be suppliedto the antenna or other utilization device in known manner.

In an alternative adjustment and manner of operation of the device ofFig. 2 as a modulator the section 6 is oriented in space at an anglewith respect to section 5 by means of the rotatable joint 60 and thepotentiometer arm may be set at the extreme right-hand end of thepotentiometer 9. The physical angle of the section 6 with respect tosection 5 may be selected as 45 degrees, for example, in which case theoutput in the absence of magnetizing current is one-half the maximumobtainable by optimum polarization. Application of modulating potentialsthrough the transformer 13 then introduces an alternating magnetizingcurrent in the solenoid 7 which rotates the plane of polarization of theWaves alternately, clockwise and counterclockwise, and therebyalternately decreases and increases the power output of section 6 inaccordance with the variations in the modulating potentials.

Fig. 3 shows the use of the element 1 in an automatic volume controlsystem or antifading device. The arrangement of parts is the same as inFig. 2 except that the potentiometer 9 and switch 12 may be dispensedwith, the magnetization being supplied by current from the fixed source,battery 8. A probe 15, which preferably projects only slightly into thewave guide so as to disturb transmission therein only slightly, is addedin the section 6 for the purpose of picking up a monitoring potentialfrom the wave therein. The probe 15 may be connected as through acoaxial transmission line 16 to a monitoring detector 17 the output ofwhich latter is connected to a resistor 18 which is serially connectedin the energizing circuit of the solenoid 7. The resistor 18 ispreferably shunted by a condenser 19 in the usual manner of a volumecontrol system. The wave guide sections 5 and 6 are preferably, but notnecessarily, similarly oriented.

In the operation of the arrangement of Fig. 3 the poten- Sil tial of thesource 8 may be selected to produce aninter-v mediate amount of phaserotation so that normally the output in the section 6 is less than themaximum output and may advantageously be made one-half the maximum. Avariable increment is added to the potential applied by the source 8 dueto the presence of a control current supplied by the monitoring detector17 and passing through the resistor 18. Should fading occur in the wavesupplied to the system through the section 5 so that the power output inthe section 6 is decreased, the amount of energy picked up by the probe15 is also decreased and the current passed back to the resistor 18 isreduced. The connections are made in such a way that the potentialdeveloped across the resistor 18 opposes that developed by the source 8so that when the monitoring current is decreased the amount of phaserotation in the device 1 is also decreased thereby shifting the plane ofpolarization more nearly into line with the direction of the probe 15and thereby increasing the power output of the section 6 to offset thetendency of the same to decrease due to the fading. Should the powerinput of section 5 increase, the effect of the described feedback systemis to increase the phase rotation in the element 1 thereby turning theplane of polarization further away from the direction of the probe 15and decreasing the output of section 6. It is evident that the systemmay be adjusted in well known manner to maintain a substantiallyconstant output in section 6 during variation of the power in inputsection 5 over a wide range. The condenser 19 functions to smooth outthe more rapid variations in the reaction of the feedback circuitthereby avoiding unnecessary temporary readjustments of the attenuationduring transient disturbances.

Fig. 4 shows an arrangement in which the element of Fig. l is employedto transform a plane polarized wave into a circularly polarized wave.

Theoretical considerations hereinbefore set forth indicate that theFaraday rotation effect is dependent upon the existence of resonantfrequencies in the magnetic material. Upon entering the magneticmaterial the impressed wave sets up two secondary waves each of which iscircularly polarized and the direction of rotation of the plane ofpolarization is opposite in the two components. When the frequency ofthe impressed wave is remote from a resonance absorption frequency thetwo secondary wave components are constrained to travel through themagnetic material with unequal velocity. The relative difference in thevelocities increases as the frequency of the impressed wave approachesthe resonance frequency. In addition, if the impressed frequency isclose to a resonance frequency, absorption takes place in the sense thatenergy from the resonant or nearly resonant wave component istransferred into heat or some other form of energy inside the materialand if the resonance is substantially complete, one of the wavecomponents may be thus completely absorbed, leaving the other componentto traverse the magnetic material and emerge at the output of the systemas a circularly polarized Wave. Instead of changing the frequency of theimpressed wave to increase absorption, it is possible to move theresonance frequency of the magnetic material by varying the magneticfield. In the microwave case, the magnetic resonance frequency isnormally much lower than the frequency of the impressed wave but theresonance frequency may be brought up into the centimeter wave range byimpressing rather large magnetizing fields. The elimination of one ofthe circularly polarized components results, of course, in a loss ofapproximately three decibels, although this is not excessive in view ofcertain advantages of securing a circularly polarized wave.

In Fig. 4 there is shown a rectangular wave guide 20 which merges into acircular section 21 containing the element 1 and surrounded by thesolenoid '7 which is connected to the battery 8.

In the operation of the system of Fig. 4 the potential of the source 8is to be made sufficiently great to bring the magnetic resonancefrequency of the element 1 into approximate agreement with the frequencyof the waves impressed upon the wave guide 20. As a result the outputwave of the section 21 is substantially circularly polarized.

Fig. 5 shows a switching system having four branches and alsoillustrates how three of these branches may be employed as in aswitching system, particularly as a device which makes it possible tosend and receive the same frequency simultaneously from the sameantenna, as in a microwave relay system for communication purposes. Thethickness of the element 1 and the potential of the source 8 are to beadjusted so as to give a 45-degree rotation of the plane ofpolarization. The output wave guide section 6 is permanently turned withrespect to the input section 5 preferably by an angle of 45 degrees inthe same direction as the rotation to be introduced by the element 1. Ahorizontal tunable probe 30 is employed on the input side of the element1, this being oriented at right angles to the (vertical) direction ofpolarization of the wave guide section 5. Another tunable probe 31 maybe located on the output side of the device 1 and is preferably orientedat an angle of 45 degrees from the vertical in a direction opposite tothe direction of rotation in the element 1. The wave guide section 6 isconnected to an antenna or horn 32 through any suitable connection. Thetunable probe 30 is connected in a suitable manner to a receiver 33, andthe tunable probe 31 to any desired system or circuit 34. Highlyconductive vanes 35 and 35' are preferably placed in the circular waveguide to reflect waves having their plane of polarization coincidentwith the plane of the vane. The spacing between each vane and theadjacent probe may be adjusted to give maximum power transfer to therespective circuits.

The operation of the system of Fig. 5 may be conveniently explained withreference to the diagram of Fig. 6. A vertically polarized wave as froma transmitter a connected to the wave guide 5 has negligible effect uponthe probe 30 because the latter is at right angles to the direction ofpolarization in the wave guide 5. The vertically polarized wave isrotated 45 degrees by the element 1 thereby bringing its plane ofpolarization at right angles to the direction of the probe 31 and intothe preferred direction for transmission through the wave guide 6 to theantenna 32. Substantially free transmission is therefore afforded fromsection 5 to antenna 32 and this condition is indicated in Fig. 6 byradial arrows labeled a and b, respectively, associated with a ring 36and an arrow 37 diagrammatically indicating progression in the sensefrom a to b. Should a wave be received from space by the antenna 32, thepolarization of the received wave in the section 6 is constrained to bein the direction indicated by b which is at right angles to the probe31. This received wave is rotated 45 degrees by the element 1 in thesame direction with respect to the magnetic field as indicated by thearrow 38 in Fig. 5 thereby bringing the wave into a polarizationagreeing with that of the probe 30 whereby the wave is conducted to thereceiver 33. Should any of this wave proceed into the rectangularportion of the section 5 it will be discriminated against as not beingin the preferred direction and may in addition be reflected by means ofa conductive vane 35. This vane is inserted at such a position that thetunable probe 30 will be at an antinode in the voltage standing wavepattern. It will be evident that the wave of polarization as representedby b is conducted through the system to the receiver in the polarityindicated by c. This transmission is indicated by the arrow 37 in Fig. 6which tendsto turn the arrow b into the direction of the arrow 0. A waveof the polarity represented by c impressed upon the probe 30in anyconceivable manner is not transmitted to the transmitter a and whenrotated 45 degrees by the element 1 is polarized in the proper directionfor transmission by the probe 31 to the circuit 34 as indicated in Fig.6 by the transition from c to d. In the above-cited example the circuit34 i and probe 31 are not used but these can be employed wheneverdesirable in other applications of the general 2 system of Fig. 5. Awave polarized in the direction of d of sign is indicated in Fig.6 bythe arrow labeled -a.

Fig. 7 is an alternative diagram representing the same information asthat given in Fig. 6 but in a slightly different form. Here, thebranches a, b,' c, and d are represented at right angles to each otherand may be re- ..garded merely as the four circuits of Fig. 5, eachconsidered without regard to its direction of polarization. The arrow 39indicates the scheme of transmission when taken in conjunction with aminus sign 40. Transmission of waves impressed at a takes these waves tocircuit b, transmission from b leads to circuit 0, transmission fromcircuit c leads to circuit d and transmission from circuit d leads witha change of sign to circuit a. It is obvious to one familiar with theart that circuits 0 and d can be coupled to the wave guide by othermeans than the coaxial tunable probes illustrated.

Fig. 8 shows a one-way transmission system embodying the invention in asimilar manner to the system of Fig. 5. The tuners 30 and 31, thereceiver 33 and the circuit 34 are omitted and the wave guide section 6may be connected to any desired load. Also resistive vanes 46 and 47 areused in place of reflecting vanes 35 and 35'.

In the operation of the system of Fig. 8, a transmitter may be connectedto the input wave guide section 5 and the output wave guide section 6may be connected to a transmission line, for example, the line being onewhich has some irregularity such as will reflect a portion of a wavesent into the line. The system of Fig. 8 operates to prevent or greatlyreduce the reaction of the reflected wave upon the transmitter. Tounderstand how this result is effected, consider a vertically polarizedwave impressed upon the wave guide section 5 by the transmitter. Thiswave undergoes a 45-degree rotation of its plane of polarization in theclockwise sense as it traverses the element 1 from left to right and isenabled thus to pass freely through the wave guide section 6 to theline. A reflected portion of the wave thus introduced into the line,returning with the same direction of polarization undergoes a furtherrotation of its plane of polarization as it passes through the element 1from right to left, thereby rendering its plane of polarization intospace quadrature with the'preferred plane of polarization of the waveguide section 5. Accordingly the reflected wave is rejected or greatlyattenuated and its reaction upon the transmitted wave is greatly reducedcompared with what it would be in the absence of means embodying theinvention.

Fig. 9 shows a switching system having four branches and generallysimilar in resulting operation to that shown in Fig. 5 except that thephase reversal indicated by the minus sign 40 in the diagram of Fig. 7is eliminated. In Fig. 9 the input guide 41 and the output guide 42 havetheir preferred directions at right angles to each other and the element1 is arranged to give a rotation of degrees. The wave guides 41 and 42lead respectively each to one arm of a wave guide junction or T joint 43and 44 respectively. These T joints are of the type shown in Fig. 7 ofUnited States Patent No. 2,445,895, issued July 27, 1948, to W. A.Tyrrell, a type of junction which is commonly referred to as a magic Tand also as a wave guide hybrid. Each of the T junctions 43 and 44 leadsto two branches, those in junction 43 being designated a and c and thosein junction 44 being designated b and d. A reversing switch 45 isprovided between the supply source 8 and the solenoid 7 for reversingthe'magnetization of the element 1 when desired. Resistive 'vanes46, 47are provided for absorbing wave components havingundesired polarizationdirections.. A'resistive vane 48 which may be adjustable is inserted inthe wave guide connection 49 between the junctions 43 and 44 to equalizethe attenuation in the two circuit branches between the junctions 43 and44. These branches comprise one branch through the element 1 and theother through the guide 49. A 90-degree permanent twist is introducedinto the wave guide 41 at 50.

The operation of the device in Fig. 9 is conveniently explained byreference to Fig. 10. This figure is the same in plan as Fig. 6. A waveentering the junction 43 from the circuit a divides in two parts at thejunctions, one part being transmitted through the element 1 and theother through the wave guide 49. The wave portion passing through theelement 1 receives a 90-degree rotation which together with the twist inthe wave guide at 50 brings this portion of the wave to the junction 44with the same polarization, and the same time phase as the wave comingthrough the wave guide 49 if the two path lengths are adjusted so thatthis phase relation obtains. These two components annul each other incircuit d but combine in circuit b in like phase. Thus, as illustratedin Fig. 10 we have substantially free transmission from a to b, thesense of progression being as indicated by the solid arrow 51. Wavesentering the junction 44 from the circuit b divide into two polarizedcomponents of like sign, one passing through wave guide 49 and the otherthrough element 1 with a 90-degree rotation. The latter component afterpassing through the twisted section 50 arrives at the junction 43 insuch phase relation to the other component as to be freely transmittedinto the circuit c and to be annulled in the circuit a. Similarly,transmissions originating in circuit c pass freely into circuit d andtransmissions originating in circuit d pass freely into circuit a asindicated in Fig. 10. Should the reversing switch 45 be operated toreverse the magnetization of the element 1 the transmissioncharacteristics of the system are reversed as indicated by the dottedarrow 52 in Fig. 10.

The element 1 when operated to produce a 90-degree rotation, as forexample in the arrangement of Fig. 9 is a device of the type which hasbeen called a gyrator, by B. D, H. Tellegen in an article entitled, TheGyrator, a New Electric Network Elemen in Philips Research Reports,volume 3, pages 81 through 101, published in 1948. Tellegen defines theideal gyrator as a passive four-pole element which is described by v =Siwhere 1 and v are the voltages respectively across the input and outputpole pairs of the device, i and i are the corresponding currents throughthe input and output pole pairs respectively, and S is a (complex)factor of proportionality. Since the coefiicients of i and of i are ofopposite sign in Equations 19 and 20, the gyrator violates the theoremof reciprocity. In simple terms this equality of coefficients but withopposite sign means that a 180-degree phase difference exists withrespect to propagation of waves through the gyrator in the twodirections. For this reason the gyrator may be termed an antireciprocaldevice and represents a special class of devices which may be termednonreciprocal because they violate the theorem of reciprocity. Thedevice of Fig. 9 may also be described as a directionally selectivephase shifter. It produces a phase diiference of 180 degrees betweentransmissions in opposite directions, so that a wave passing twicethrough the element 1 in opposite directions is subjected to a180-degree total phase shift, or what is the same thing, a phasereversal. In contrast to this device, a reciprocal phase shifter is onewhich, when traversed by a wave first in one direction and then in theopposite direction produces no net phase shift.

Fig. 11 shows an arrangement whereby the system of the invention may beused with massive pole pieces or a magnetic core structure such as shownat 55 which may be a permanent magnet or the core of an electromagnet.The wave guide containing the element 1 is inserted through holes 56, 57in the structure 55 the axes of these holes being preferably parallel tothe direction of the magnetic flux and may be connected between atransmitter and a load in any manner and with any relative orientationof the rectangular wave guide portions such as shown in illustrations ofany of the other embodiments of the invention.

It will be evident that the transition members 2 and 3 mayadvantageously be made of ferrite of the same composition as the element1, thereby tending further to reduce reflections, while the transitionmembers themselves then also contribute to the amount of Faradayrotation produced. Also the transition members 2 and 3 and the element 1may be combined into a single member.

The nickel-zinc ferrite element which was made as described hereinbeforein reference to Fig. 1 was of cylindrical form, having a length of 1.4centimeters and a diameter substantially fitting the inside of acircular wave guide of inside diameter 0.9 inch. Fig. 12 shows theresults of measurements made upon this element. The abscissae representstrength of the applied magnetic field in oersteds while the ordinatesrepresent the resulting Faraday rotation expressed in degrees percentimeter thickness of the material traversed by the impressed waves.From the curve it can be ascertained that for a thickness of 1.4centimeters of this material an angle of rotation of 45 degrees isobtained by applying a magnetic field of approximately 320 oersteds andan angle of degrees is obtained by applying a magnetic field ofapproximately 1040 oersteds. Saturation was reached at approximately1040 oersteds. Saturation was reached at approximately 96 degrees bymeans of an applied field of approximately 1350 gauss.

In any of the embodiments of the invention illustrated, the element 1may be replaced by a container of gas or liquid or by material in solidform other than ferrite, the choice of material being limited only bythe presence of a proper resonance absorption frequency for the materialnot too far removed from the desired frequency of the incident waves.Among suitable materials are gases trapped in a clathrate molecularstructure, as gas so trapped is equivalent to ordinary gas at enormouspressure and great density and is capable of producing large Faradayrotations in a wave guide of reasonable length. Since it has beendemonstrated that the rotation is proportional to the magnetization andnot to the applied magnetic field, it is also obvious that the solenoidcan be dispensed with if a suitable ferromagnetic material is used whichcan be permanently magnetized.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. In combination, two wave guide hybrid junctions each having twomutually conjugate branches and two mutually symmetrical branches, twowave guide transmission paths each connecting one of the mutuallysymmetrical branches of one said hybrid junction with one of themutually symmetrical branches of the other said hybrid junction, one ofsaid transmission paths including a twisted length of wave guide forintroducing a reciprocal rotation of polarization direction of linearlypolarized waves amounting to 90 degrees, said one transmission path alsoincluding nonreciprocal polarization direction rotating means producinga nonreciprocal rota tion of 90 degrees.

2. A selective transmission system comprising two wave guide hybridjunctions each said junction comprising two mutually conjugate branchesof rectangular cross section V v 13 a and two mutually symmetricalbranches of rectangular cross section, a first wave guide transmissionpathconnecting one of the mutually symmetrical branches of one of thesaid hybrid junctions with one of the mutually symmetrical branches ofthe other said hybrid junction, a second wave guide transmission pathconnecting the remaining mutually symmetrical branches of the respectivehybrid junctions, saidsecond transmission path comprising a twistedlength of wave guide of rectangular cross section providing a reciprocalrotation of 90 degrees in the direction of polarization of planepolarized waves and nonreciprocal means providing a nonreciprocalrotation of 90degrees. t

3. A system in accordance with claim 2, with attenuating means includedin said first wave guide transmission path for equalizing thetransmission loss in said first and second transmission paths. c

4. A selective transmission system comprising a first pair ofpolarization-direction selective mutually conjugate circuit branches aand c, a second pair of polarizationdirection selective mutuallyconjugate circuit branches b and d, and a pair of polarization-directionselective transmission paths each connecting branches a and c withbranches b and d, one said transmission path comprising a 90-degreetwisted section providing a reciprocal 90- degree rotation of thedirection of polarization of linearly polarized waves passingtherethrough and further comprising a 90-degree nonreciprocalpolarization-direction rotating device.

5. A selective transmission system comprising first and secondfour-branch power dividing networks, each of said branches having thebranches thereof arranged in pairs with the branches comprising one pairbeing conjugate to each other and in coupling relation to the branchesof the other pair, a first wave transmission path connecting a branch ofone pair of branches of said first network to a branch of one pair ofbranches of said second network, a second wave transmission pathseparate from said first path connecting the other branch of said onepair of said first network to the other branch of said one pair of saidsecond network, an input branch on said first fourbranch network and anoutput branch on said second four-branch network, means including amagnetized ele ment of gyromagnetic material interposed in at least oneof said paths for shifting the phase of wave energy propagating in saidone path in the direction from said first to said second network suchthat the differential phase angle between the wave energy transmittedfrom said input branch to said output branch along said first and secondtransmission paths is 180 degrees said differential phase angle beingnonreciprocal and capable at the same time of being zero degree for waveenergy transmitted from said output branch to said input branch alongsaid first and second transmission paths.

6. A selective transmission system comprising first and secondfour-branch power dividing networks, each of said networks having thebranches thereof arranged in pairs with the branches comprising one pairbeing conjugate to each other and in coupling relation to the branchesof the other pair, a first wave transmission path connecting a branch ofone pair of branches of said first network to a branch of one pair ofbranches of said second network, a second wave transmission pathseparate from said first path connecting the other branch of said onepair of said first network to the other branch of said one pair of saidsecond network, means including a magnetized element of gyromagneticmaterial interposed in at least one of said paths for shifting 180degrees the relative phase angle of wave energy transmitted along saidfirst path with respect to wave energy transmitted along said secondpath from one branch of the other 1 pair of said first network to onebranch of the other pair 2 of said second network, such relative phaseshift being nonreciprocal and capable at the same time of being equal tozero degrees for wave energy transmitted along jugate to each other andin coupling relation to the branches of the other pair, a first wavetransmission path connecting a branch of one pair of branches of saidfirst network to a branch of. one pair of branches of said secondnetwork, a second wave transmission path separate ,1 from said firstpath connecting the other branch of said one pair of said first networkto the other branch of said one pair of said second network, meansincluding a magnetized element of gyromagnetic material interposed in atleast oneof said paths for shifting the phase of wave energy propagatingin said-first path with respect to wave energy propagating in saidsecond path by a phase angle in one direction of X degrees, said phaseshift being nonreciprocal and capable at the same time of shifting thephase of wave energy propagating in said first path with respect to waveenergy propagating in the said second path in the reverse direction by avalue of X- degrees.

8. A selective transmission system comprising first and secondfour-branch power dividing networks, each of said branches having thebranches thereof arranged in pairs with the branches comprising one pairbeing conjugate to each other and in coupling relation to the branchesof the other pair, a wave transmission path connecting a branch of onepair of branches of said first network to a branch of one pair ofbranches of said second network, a separate wave transmission pathconnecting the other branch of said one pair of said first network tothe other branch of said one pair of said second network, a magnetizedelement of gyromagnetic material interposed in at least one of saidpaths for shifting the phase of wave energy propagating in said one pathin the direction from said first to said second network with respect towave energy propagated in said direction in the other of said paths,said phase shift being at the same time nonreciprocal with respect towave energy propagating in the reverse direction in said path.

9. A selective transmission system comprising first and secondfour-branch power dividing networks, each of said branches having thebranches thereof arranged in pairs with the branches comprising one pairbeing conjugate to each other and in coupling relation to the branchesof the other pair, a wave transmission path connecting abranch of onepair of branches of said first network to a branch of one pair ofbranches of said second network, a separate wave transmission pathconnecting the other branch of said one pair of said first network tothe other branch of said one pair of said second network, a magnetizedelement of gyromagnetic material interposed in at least one of saidpaths for shifting the phase of wave energy propagating in said one pathin the direction from said first to said second net work 180 degreeswith respect to wave energy propagated in said direction in the other ofsaid paths while at the same time preserving an in-phase relationbetween wave energy propagated in the direction from said second to saidfirst network in said paths.

10. The transmission system according to claim 7 wherein saidgyromagnetic material is magnetized in a direction parallel to saiddirection of propagation whereby an antireciprocal rotation of thepolarization of said wave energy is produced, and means for producing areciprocal rotation of said polarization interposed in said pathadjacent to said material.

11. The transmission system according to claim 10 wherein a twistedsection of wave guide comprises said means for producing said reciprocalrotation and is in- 15., terposed in said path adjacent to saidgyromagnetic material.

12. A multipath transmission system comprising two waveguidetransmission paths having first and second interconnections, anantireciprocal rotator of wave energy interposed in one of said pathsbetween said interconnections, said antireciprocal rotator beingadjusted for producing a rotation of the polarization of said waveenergy through an angle of 90 degrees, a twisted length of waveguideincluded in said one path between said antireciprocal rotator and one ofsaid interconnections for providing a reciprocal rotation of thepolarization of said wave energy through an angle of 90 degrees, saidreciprocal rotation being cumulative with said antireciprocal rotationfor propagation from said first to said second interconnection to shift180 degrees the relative phase between energy in said paths at saidsecond interconnection with respect to that existing at said firstinterconnection, said reciprocal rotation cancelling said antireeiprocalrotation for propagation from said second to 16 said firstinterconnection to preserve the relative phase shift between energy insaid paths at said first interconnection with respect to that existingat said second interconnection.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Ragan: Microwave Transmission Circuits, vol. 9, M. I. T. Rad.Lab. Series, McGraw-Hill, 1948, page 208 relied on.

